Thermally stimulated luminescence properties of BaY 2 F 8 :Ce Crystals

Wavelength resolved thermally stimulated luminescence (TSL) measurements were performed on BaY 2 F 8 :1.8 v mol% Ce crystals after X-ray irradiation at 10 v K and at 300 v K, in order to obtain preliminary information about both trap levels and recombination centres. After irradiation at 10 v K, the TSL glow curve shows the presence of a strong peak at 50 v K, together with additional structures at approximately 20 and 170 v K. The TSL spectrum is dominated by the characteristic doublet emission due to transitions from the lowest energy level of the 5d configuration to the spin-orbit split 2 F ground state of Ce 3+ . Above RT, the glow curve exhibits a peak at 60 v C, whose spectrum is again dominated by Ce 3+ emission. The TSL emission is in accordance with radio-luminescence (RL) spectra performed in the 10-300 v K region. Moreover, RL spectra at temperatures lower than 200 v K display an additional weak high energy band at around 4.5 v eV assigned to host lattice transitions.     

Radiation induced colour centers and damage in YAlO 3 :Ce and YAlO 3 :Ce,Zr scintillators

Radiation induced absorption spectra (irradiation up to 500 v Gy by 60 Co) and TSL of a set of YAP:Ce crystals co-doped by Zr 4+ were investigated. Positive effect of Zr 4+ co-doping was found, namely reduction of both the defect creation and presence of TSL active traps. Up to three absorption bands were observed in the radiation induced absorption spectra. They probably have different origin - from colour centers in the green part of the spectra to change of the Ce 3+ valency in the near UV.     

Trap levels in Y-aluminum garnet scintillating crystals

Point defects acting as trap levels were investigated on undoped, Ce- and (Ce, Si)-doped Y₃Al₅O₁₂ (YAG) crystals by TSL measurements performed over a wide temperature range (10–800 K). Below room temperature, a composite glow curve was observed, whose intensity strongly increased after Ce doping. Moreover, Ce doping introduced new trap levels giving rise to glow peaks in the 100–200 K range. On the other hand, Si co-doping did not influence the low T glow curve in a significant way. The spectral emission of the TSL was found to be governed by the Ce³⁺ 5d–4f radiative transition, while defect related higher energy emission bands were detected only in the undoped crystal. Above RT, the glow curve was found to be much more influenced by Si co-doping since a strong increase of a glow peak at about 250°C was noticed. Scintillation time decays of Ce- and Ce,Si-doped samples are also reported and compared with TSL data. The significance of the results and the potential impact of defect states on the scintillation properties are discussed.     

Magnetic resonance investigations of LaCl 3 :Ce 3+ scintillators

In undoped and Ce 3+ -doped LaCl 3 EPR has been detected in the X-ray luminescence (XL-EPR) in K-band (25 v GHz) at 1.5 v K. Two excited triplet states with different EPR parameters and spectral shapes could be separated, both triplet states have been attributed to "out-of-plane" self-trapped excitons (STE) in LaCl 3 . No EPR signals of V K centres (self-trapped holes) could be detected in undoped or Ce 3+ -doped LaCl 3 after X-irradiation at low temperatures. X-irradiation of undoped LaCl 3 creates also an EPR spectrum which can be tentatively attributed to F-type defects. The scintillation mechanism is discussed.     

Site symmetry and crystal field splittings of Ce 3+ in LiLuF 4 and LiSr 0.8 Ca 0.2 AlF 6

Electron paramagnetic resonance (EPR) spectra of Ce 3+ in LiLuF 4 (LLF) and LiSr 0.8 Ca 0.2 AlF 6 (LSCAF) observed at low temperatures (<20 v K) show that the Ce 3+ centers have tetragonal and pseudo-trigonal symmetry with ( g , g )=(2.751, 1.467) and (2.14, 0.80), respectively. The EPR lines of Ce 3+ in LSCAF are inhomogeneously broadened by different Ce 3+ configurations which correspond to the random occupation of the second nearest neighbor cation sites by Ca 2+ and Sr 2+ and/or charge compensators. These EPR results indicate that the Ce 3+ centers in LLF and LSCAF are associated with substitution of Ce 3+ for Lu 3+ with eight-fold coordination and Sr 2+ /Ca 2+ with six-fold coordination, respectively. Fourier transform infrared absorption has been used to measure transitions between the 2 F 5/2 ground state and the 2 F 7/2 multiplet of Ce 3+ in LLF at 10 v K. The optical absorption and luminescence of Ce 3+ :LLF and Ce 3+ :LSCAF in the UV range are strongly polarized. The energy levels and polarization dependence of the optical transitions can be accounted for in terms of the crystal field potential experienced by the Ce 3+ ions under the action of the different point group symmetries.     

Congruent Sr 0.61 Ba 0.39 Nb 2 O 6 doubly doped with Ce and Cr: PHoto- and thermoluminescence investigations

Congruent Sr x Ba 1 m x Nb 2 O 6 (SBN, x=0.61) doped with Ce or Cr ions exhibits enhanced photorefractive properties and new spectral features like increased red sensitivity. Here special emphasis is placed on the luminescence features of doubly doped Ce+Cr SBN crystals. The luminescence excitation and emission spectra combined with the absorption of the impurities allow to draw conclusions about the origin of the charge carriers und their recombination. The well separated thermo-luminescence peaks detected and their spectral line shape in emission point to specific recombination processes following the thermal liberation of light-induced electron trapping centers: Nb 4+ polarons and VIS-centers created at low temperature under light irradiation. The thermal activation energy for the hopping motion of Nb 4+ polarons and of VIS-centers are estimated to be 0.18 - 0.02 v eV and 0.30 - 0.05 v eV respectively. Possible excitation and recombination mechanisms in SBN:Ce+Cr are discussed.     

Optical and scintillating properties of Ce:Li(Y,Lu)F4 single crystals

We have investigated the optical and scintillating properties of Lu co-doped Ce:LiYF₄ single crystals with various Lu content. In the transmittance and absorption spectra, the absorption peaks at 243 nm get systematically red shifted in contrast to the peaks at 197 and 200 nm which get blue shifted with the increase in Lu content. At the same time, emission peaks at 306 nm and 200 nm under 295 nm excitation also get red shifted. The decay time of Ce:Li(Y,Lu)F₄ crystals under 295 nm excitation is found to be faster than that of Ce:LiYF₄ and Ce:LiLuF₄ crystals. The alpha-peak positions in the pulse-height spectra and decay times of crystals under alpha-ray irradiation are found to vary with the Lu content.     

Simultaneous three-photon-excited violet upconversion luminescence of Ce3+:Lu2Si2O7 single crystals by femtosecond laser irradiation

We found that Ce3+:Lu2Si2O7 single crystals could be excited at 800 nm by using a femtosecond Ti:sapphire laser. The emission spectra of Ce3+:Lu2Si2O7 crystals were the same for one-photon excitation at 267 nm as for excitation at 800 nm. The emission intensity of Ce3+:Lu2Si2O7 crystals was found to depend on the cube of the laser power at 800 nm, consistent with simultaneous absorption of three 800 nm photons. The measured value of the three-photon absorption cross section is sigma'3=2.44x10(-77) cm6 s2.     

Electron–hole recombination luminescence in LiYF4:U single crystal

Photoluminescence (PL), photostimulated luminescence (PSL), thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) studies were carried out on LiYF₄:U⁴⁺ and pure LiYF₄ crystals. The PL and EPR investigations have identified the presence of Eu³⁺, Tb³⁺ and Gd³⁺ ions in both of these crystals possibly due to their existence in the starting materials. The luminescence observed during afterglow, PSL and TSL revealed that emission occurs at wavelength positions 382, 413, 437 and 544 nm, which are characteristic of Tb³⁺ ions. The present investigations using PSL and TSL in combination with PL studies before and after gamma irradiation have revealed that selective energy transfer to Tb³⁺ ions occurs during electron–hole recombination processes like PSL and TSL. Even though other luminescent ions (U⁴⁺ and Eu³⁺) are present in the system and U⁴⁺/U³⁺ ions are participating in electron capture/release processes, the selective energy transfer results in Tb³⁺ ions acting as luminescence centers.     

Cathodoluminescence of Lu 2 O 3 :Tb

Spectroscopic properties of Lu 2 O 3 :Tb pellets sintered at 1000 v C or 1700 v C were investigated. The sintering temperature did not influence the optically excited emission and excitation spectra of the materials. However, the treatment temperature did influence the emission under cathode-rays stimulation. While the specimens sintered at 1000 v C showed only luminescent lines above 480 v nm resulting from a radiative relaxation of the 5 D 4 level, the samples heated at 1700 v C did show some luminescence in the blue region resulting from 5 D 3 M 7 F J emission. The blue emission could be recorded exclusively upon cathode-rays irradiation.     

Photoluminescence and EPR studies on gamma irradiated Ce doped potassium halide single crystals

Electron Paramagnetic Resonance(EPR), Photoluminescence(PL), Thermoluminescence (TL) and other optical studies of γ-irradiated KBr, KCl:Ce³⁺ single crystals. Cerium when doped into the KBr, KCl is found to enter the host lattice in its trivalent state and act as electron trap during γ-irradiation, thereby partially converting itself to Ce²⁺. The Photoluminescence(PL) spectra of both KCl and KBr crystals doped with Ce exhibit the strong blue emissions of Ce corresponding to ⁵d(²D)→²F_5/2 and ⁵d(²D)→²F_7/2 transitions. The defect centers formed in the Ce³⁺ doped KBr and KCl. Crystals are studied using the technique of EPR. A dominant TL glow peak at 374, 422 K and KCl:Ce³⁺ at 466, 475 K is observed in the crystal. EPR studies indicate the presence at two centers at room temperature. Spectral distribution under the thermoluminescence emission(TLE) and optically stimulated emission(OSL) support the idea that defect annihilation process to be due to thermal release of F electron in KBr, KCl:Ce³⁺ crystals. Both Ce³⁺ and Ce²⁺ emissions were observed in the thermoluminescence emission of the crystals.     

Optical spectroscopy of Ce 3+ ions in BaY 2 F 8 single crystals

In the present work we report on the spectroscopic properties of the Ce 3+ ion in BaY 2 F 8 single crystals. The absorption and excitation spectra of the emission centered at 340 v nm have been measured in the temperature range 15-300 v K. The 340 v nm emission consists of two broad partially overlapping bands, peaking at 324 and 347 v nm (at 15 v K), respectively. The full width at half maximum is about 0.5 v eV at room temperature. The absorption spectrum of the lowest in energy component of the f M d transition of Ce 3+ reveals at low temperature a marked vibronic structure. High resolution (0.02 v cm m 1 ) Fourier transform infrared spectroscopy in the wave number range 500-5000 v cm m 1 and in the temperature range 9-300 v K has been exploited to monitor the f level splitting. The absorption transitions from the three Stark components of the 2 F 5/2 manifold to the four of the 2 F 7/2 one, have been monitored in the wave number range 2000-3400 v cm m 1 . The wave number separation at 9 v K between the lowest level of the ground 2 F 5/2 manifold and lowest one of the 2 F 7/2 manifold is found to be 2197.47 v cm m 1 in good agreement with the splitting detected between the two components of the d M f emission.     

Deep trapping states in cerium doped (Lu,Y,Gd)3(Ga,Al)5O12 single crystal scintillators

We study deep trapping states in Ce³⁺-doped garnet crystals with the composition (Lu,Y,Gd)₃(Ga,Al)₅O₁₂, recently shown as having remarkably high light yield. We use thermally stimulated luminescence (TSL) technique above room temperature and determine the composition Gd₃Ga₃Al₂O₁₂ as the host showing the lowest concentration of traps. This host consistently manifests very low afterglow comparable to that of the standard BGO crystal. We also perform TSL glow peak analysis based on the initial rise technique to evaluate trap depth and other characteristics associated with TSL peaks.     

Thermoactivated spectroscopy of heterovalent impurity traps in CdWO4

Abstract

Recombination luminescence emission spectra, TSL and trap spectra estimated by fractional glow technique (FGT), in nominally pure and Li-, Bi- and Ho-doped CdWO₄, crystals are reported. According to the investigations by FGT heterovalent impurities Li, Bi and Ho causes localized electronic states which act as traps for charge carriers. It is shown that TSL results in emission of known blue-green luminescence band by emptying of the Li⁺-related traps in CdWO₄-Li and yellow luminescence band by emptying of the Bi³⁺-related traps in CdWO₄-Bi. It is proposed that blue-green and yellow luminescence occur by recombination correspondingly of free holes and free electrons at different intrinsic tungstate group related luminescence centers.     

Thermoluminescence of nominally pure KTaO 3 crystals

After an exposure to ultraviolet light ( u <350 v nm) at 12 v K, weak thermoluminescence of nominally pure KTaO 3 crystals was observed within the temperature region from 13 to 65 v K for the first time. An analysis of the glow curves of integral thermoluminescence revealed five glow peaks with markedly sample-dependent intensities. Three glow peaks near 26, 31, and 58 v K at the heating rate of 0.155 v K/s correspond to thermoluminescence spectrally very similar to broad-band visible photoluminescence. The glow peaks near 26 and 31 v K were assigned to the electron release from photoinduced Ta 4+ -OH m and Ta 4+ - V O centers and the glow peak near 58 v K to the hole release from photoinduced O m centers. The glow peaks near 34 and 41 v K are connected with the structureless emission band peaking near 714 v nm at 15 v K that was observed in the emission spectra of KTaO 3 crystals for the first time.     

Influence of the light illumination on thermoluminescence properties of PbWO4

The thermally stimulated properties of PbWO₄ in the temperature range 90–400 K have been investigated on undoped and terbium-doped crystals after X-ray irradiation at 90 K. Doping with terbium changes the concentration of shallow traps, which are responsible for retrapping free electrons and holes. Light illumination can change the distribution of the traps. The optically stimulated luminescence is observed. The influence of light illumination on the TSL curves and emission properties is studied. The possible mechanism of TSL phototransformation is discussed.     

Luminescence and point defects in cerium-doped lutetium scandium orthoborate crystals

Cerium-doped lutetium scandium orthoborate single crystals as potential scintillation materials were grown by the floating zone (FZ) method for the first time and the Czochralski (Cz) grown crystal was used for comparison. In this paper, the representative composition Lu_0.5Sc_0.5BO₃:0.5at%Ce (abbreviated as LSBO:Ce) was selected as the research target. The phase structure of FZ-grown LSBO:Ce crystal was characterized by X-ray diffraction and its optical properties were investigated using photoluminescence emission (PL), excitation (PLE), radioluminescence (RL) spectra. The defect properties of LSBO:Ce crystals were studied by thermoluminescence (TL) as a function of temperature (300–600 K). The glow curve of FZ-grown LSBO:Ce crystal shows three TSL peaks, at 350, 400, 552 K, corresponding to a trap depth of 0.99, 1.14 and 1.50 eV, respectively. Based on UV-irradiation TL measurements and air-annealing experiments, the former two kinds of traps could be ascribed to the radiation-induced defects and the third kind originated from the oxygen vacancies formed during crystal growth. These defects also existed in the Cz-grown LSBO:Ce crystal.     

Luminescence and thermally stimulated recombination processes in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>:Ce<Superscript>3+</Superscript> crystals

The luminescence and thermally stimulated recombination processes in lithium borate crystals Li₆Gd(BO₃)₃ and Li₆Gd(BO₃)₃:Ce have been studied. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence), temperature dependences of the intensity of steady-state X-ray luminescence (XL), and thermally stimulated luminescence (TSL) spectra of these compounds have been investigated in the temperature range of 90–500 K. The intrinsic-luminescence 312-nm band, which is due to the ⁶ P _J → ⁸ S _7/2 transitions in Gd³⁺ matrix ions, dominates in the X-ray luminescence spectra of these crystals; in addition, there is a wide complex band at 400–420 nm, which is due to the d → f transitions in Ce³⁺ impurity ions. It is found that the steady-state XL intensity in these bands increases several times upon heating from 100 to 400 K. The possible mechanisms of the observed temperature dependence of the steady-state XL intensity and their correlation with the features of electronic-excitation energy transfer in these crystals are discussed. The main complex TSL peak at 110–160 K and a number of minor peaks, whose composition and structure depend on the crystal type, have been found in all crystals studied. The nature of the shallow traps that are responsible for TSL at temperatures below room temperature and their relation with defects in the lithium cation sublattice are discussed.     

Cr-related centers in Gd3Ga5O12 polycrystals

The luminescence excitation spectra, emission spectra under photo- and X-ray excitation, luminescence decay kinetics and thermostimulated luminescence (TSL) of Gd₃Ga₅O₁₂ garnet (GGG) polycrystalline samples have been investigated. It was established that the spectrum of Cr³⁺ ion emission were present in all TSL peaks. The activation energies of traps that are responsible for appearance of TSL in the region 295-600 K were estimated. It is shown that delocalization of electrons from the Cr³⁺e traps leads to the appearance of thermoluminescence (TL) glow peak at 390 K. The nature of other TSL peaks is discussed. The influence of visible light on the TSL intensity of the preliminary X-ray-irradiated samples is shown.     

Luminescence spectra of doped and undoped lead tungstate crystals

TL spectra of undoped lead tungstate crystals exhibit glow peaks at 30 v K and 85 v K centred at 440 v nm, plus a peak at 50 v K at 530 v nm in an annealed sample. Annealing adds a 170 v K peak. Trivalent dopants of La 3+ and Y 3+ reduce the green luminescence, and Nb 5+ introduces a peak near 100 v K centred at 530 v nm; Sb introduces features between 40 v K and 90 v K and 150 and 180 v K. The luminescence emissions around 50 v K may be attributed to complex intrinsic defect centres, including (WO 4 ) m 3 . Of the four dopants studied in the present research, Sb +5 has the highest luminescence intensity. CL spectra show interesting anomalies near 170 v K which are linked to a phase change of water/ice nanoparticles trapped at dislocations.